This invention relates to an apparatus for testing sheet material, in particular bank notes, comprising a device for transporting the sheet material in a defined direction, at least one pair of transducers with a transmitter which exposes the sheet material to soundwaves and a receiver which detects the sound fraction transmitted through the sheet material, transmitter and receiver being disposed obliquely to each other with respect to the transport plane.
An apparatus of the abovementioned kind is known e.g. from EP 0 167 010 A2. A sound transmitter emits ultrasonic waves onto the moving sheet material, the sound fraction transmitted through the material under test being detected by a receiver. The detected sound intensity is evaluated as a measure for determining the thickness and/or weight per unit area of the sheet material. Transmitter, material under test and receiver are disposed obliquely to each other such that the sound fractions reflected on said elements are directed out of the beam path between transmitter and receiver (measuring path). Additional devices, such as sound traps or sound mirrors, ensure that the removed sound fractions cannot return to the measuring path. However, the use of such additional devices is relatively elaborate and requires additional room.
The invention is based on the problem of proposing an apparatus for testing sheet material that permits a good signal yield with low technical effort and is of compact construction. This problem is solved according to the invention by the features stated in the characterizing parts of the independent claims.
The basic idea of the invention is that the soundwaves emitted by a transducer are passed into a transport channel in which the sheet material to be tested is moved during acoustic irradiation, second sound fraction of sound waves reflected by the sheet material being limited in its propagation by the transport channel such that said second sound fraction are directed away from the receiver during irradiation of the sheet material. Preferably, the transducer produces a sound lobe with a small aperture angle, the soundwaves emitted by the transducer not impinging directly on the sound-reflecting inner surfaces of the transport channel. Provision of a transport channel avoids interference in the measuring signal by reason of vagrant soundwaves which are not to contribute to measurement. The transport channel permits this to be attained with low technical effort since no additional devices such as sound traps or sound mirrors are required. Further, since the transport plane is free from sound-absorbent materials the transport channel can be made so narrow that the transmitter and receiver can be disposed relatively close together. Since a major part of the soundwaves emitted by the transmitter is reflected by the material under test and only a small part (in the range of a few percent) passes through the material under test in a transmission measurement, the signal yield of the measurement is improved by the small distance between transmitter and receiver. This increases the signal-to-noise ratio when background noise is constant, thereby facilitating evaluation of the measuring signal.
Preferably, testing is effected over the total width of the sheet material, the sheet material moved in the longitudinal direction being scanned in a plurality of measuring tracks perpendicular to the transport direction. This makes it possible e.g. to ascertain adhesive tape on bank notes or tears or holes in the bank notes or else concertina folds or dog ears, which is important for testing the state of bank notes. With a multitrack testing apparatus the transducer pairs (transmitter and receiver) are disposed at a great distance apart in comparison with the distance between the inner surfaces, i.e. the height of the transport channel. The distance between the inner surfaces of the transport channel is preferably selected so that the sound fraction reflected by the sheet material is reflected several times between an inner surface of the channel and the material, the sound intensity decreasing greatly with an increasing number of reflections, thereby making the power of the sound fraction which can pass into an adjacent measuring track negligible. Since this condition depends on many factors, for example the surface quality of the inner surfaces of the transport channel, the maximum angle of emergence of the sound or the reflection factor of the sound-absorbent material, this condition can only be stated as a guideline, whereby the distance between two adjacent measuring tracks should be at least five times as great as the transport channel height.
The transducer is held in position in the housing by an elastic element such that the oscillating body when suitably excited undergoes almost no attenuation through the mounting of the housing and the sound axis of the transmitter is nevertheless aligned exactly with the receiver. One can thus achieve a sufficient signal yield even using small transducers with lower sound power. With multitrack testing of the sheet material, the transducers are preferably disposed on a common elastic element made e.g. of a foam material. For example the elastic element is disposed on the side facing away from the transducers on a board, the connections of the transducers being connected with the board via electric conductors guided e.g. through the foam material. The transducers are surrounded by a housing which defines the positions of the transducers, the housing being connected detachably with the board. This permits the ultrasonic transducers to be replaced in simple fashion, thereby improving ease of maintenance.